Specific numbers aside, one possible reason is they want to increase adoption to gain user volume, in order to have an effect on the larger ecosystem.
Once you have non-trivial network effects, you could continue to influence the ecosystem (see: MSIE, Firefox in its early days, and Google Chrome). There are probably multiple paths to this. This is one.
Influence isn’t all about the raw numbers - if yours is the browser of choice for developers, that’s going to give you a stronger voice than just being the fourth best browser. Think about Twitter - by all accounts, Twitter’s user numbers were dwarfed by the other networks, but they punched way above their weight because the entire political policy making and reporting apparatus was on there.
